Apparatus and method for low pressure sand casting

ABSTRACT

A process for counter gravity sand casting including providing precision cores in a sand mould supported in a casting machine for rotation about a horizontal axis through the center of the mould, providing primary casting risers fed by a launder section, a pressure riser connected to the launder section and the primary risers, rising upwardly beside the mould, whereby on rotation a the molten metal in the pressure riser will maintain a internal constant pressure in the mould until the casting risers are upper most, so as to maintain the internal pressure during the cooling of the mould.

The present invention relates to methods of casting light metals andmore particularly to the use of a riser that retains pressure on theliquid casting during the sand moulds rotation through 180°. Moreparticularly, the present invention is directed to an improved processand method of casting aluminium with the use of precision sand andcounter gravity filling of moulds followed by 180° rotation of the mouldto put the risers and feed metal on top of the casting.

BACKGROUND TO THE INVENTION

For purposes of explanation, reference will be made to the use of thepresent invention with respect to the casting of motor cases (engineblocks). It should be understood by those of ordinary skill in the artthat the invention is not limited to use in casting engine blocks andcan be used in the casting of other products. Cylinder Blocks havetraditionally been manufactured as a casting in Cast Iron. RecentlyAluminium alloys have become the material of choice for cylinder blocks.While the weight and thermal conduction rates of aluminium have bigadvantages over cast iron, the actual casting of liquid aluminium isproblematic. Conventional gravity pouring of aluminium alloys results inturbulent flow and the manifestation of oxides dispersed through thecasting. These oxides often become the failure points for the casting inservice. To overcome this problem with oxide formation, counter gravityfilling of the liquid aluminium from the bottom of the mould has becomethe preferred method of casting. The major problem with counter gravityfilling of moulds is the slow production rates, the mould filling systemeither low pressure or electromagnetic pump need to retain pressureuntil the casting has solidified, which for a cylinder block can be upto eight minutes. While the counter gravity filling is desirable, thesolidification time and subsequent low productivity are not. To overcomethis low productivity problem, methods of disconnecting the mould fromthe filling system and rotating the mould while the casting is stillliquid have been developed. Once the mould and casting are rotatedthrough 180° the risers which supply liquid metal during thesolidification phase of the cast process are on top of the casting andgravity feed the required liquid metal into the contracting casting.

A major problem has been the differential pressures created in theliquid casting during rotation, which can result in castingimperfections; the present invention shows how to overcome this problem.

BRIEF STATEMENT OF THE INVENTION

Thus there is provided according to the invention a process for countergravity sand casting including providing precision cores in a sand mouldsupported in a casting machine for rotation about a horizontal axisthrough the center of the mould, providing primary casting risers fed bya launder section, a pressure riser connected to the launder section andthe primary risers and rising upwardly beside the mould, whereby onrotation the molten metal in the pressure riser will maintain a internalconstant pressure in the mould until the casting risers are upper mostto maintain the internal pressure during the cooling of the mould.

In a further form, the invention may be said to reside in sand mould fora counter gravity filling casting operation including primary castingrisers fed by a launder section, a pressure riser connected to thelaunder section and the primary risers, rising upwardly beside themould.

Preferably the mould is connected to a roll over fixture and castingmachine via at least one chill, which forms part of the mould.

In a further form, the invention may be said to reside in a process forcounter filling a sand casting including primary casting risers fed by alaunder section, and a pressure riser connected to the launder sectionand the primary riser, rising upwardly beside the mould, including thesteps of filling the mould using mould filling means, sealing the mouldvia mould sealing means, and rotating the mould via mould rotationmeans.

Preferably, the risers are fed by a launder section, and a pressureriser connected to the launder section and the primary riser, risingupwardly beside the mould, including the steps of filling the mouldusing mould filling means, sealing the mould via mould sealing means,and rotating the mould via mould rotation means.

Preferably, the molten metal is fed into the mould void at its lowermostposition.

Preferably, a PLC controlled closed loop feedback is used to control theliquid fill rate to the mould.

Preferably, when the mould filling means has filled the mould, an inputfrom a metal level sensor will direct the mould sealing means to push asand slide into position so as to disengage the mould from the metalfilling system.

Preferably, the rotation means will rotate the mould through 180° whilethe metal is still liquid, such that the pressure riser maintains aconstant positive pressure on the mould during the roll.

BRIEF DESCRIPTION OF THE INVENTION

FIG. 1 Shows a schematic of the mould on the casting machine duringfill, and

FIGS. 2–6 Show the mould and liquid casting in isolation during theroll, the pressure riser position is always in a higher elevation thanthe casting until the primary risers are on top of the casting at rollcomplete.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIGS. 1 to 6. There is an apparatus for countergravity filling a mould 2 including a tower or reverbatory meltingfurnace, in which the aluminium is melted. From here it is thenlaundered to a holding furnace 1, from which the liquid metal is pumpedeither by an electromagnetic or pneumatic pump system 12, to the mould 2via the metal filling system launder section shown at 14.

With reference to the drawings the mould 2 is supported in the castingmachine by the H13 steel chill section 16, for rotation about an axis(not shown) passing through the approximate center of the mould. Thechill section is formed into the mould. The primary casting risers 20extend upwardly into the mould from runners 4 fed from the laundersection 14. Also connected to the launder section 14 is a vertical riser10 extending upwardly outside the mould, the riser in the positionsshown in FIG. 1 having a height equal to the height of the mould. Inthis way during the casting operation the riser is also filled.

The electromagnetic pump 12 pumps the liquid metal into the laundersection 14 to fill the mould during the casting operation. Thiselectromagnetic pump 12 is controlled by a PLC 22. Load cells 18 areprovided to weigh the liquid filling the mould, the weight of the liquidfilled mould being known, the load cells also being connected to the PLC22, which is implementing fuzzy logic control of the system.

When the mould has been filled (determined using the load cells 18) thePLC 22 controls the closure of a sand slide (not shown) into positionthat will disconnect the mould 2 from the filling section. As shown inFIGS. 2 to 5 as the mould is then rotated, the pressure riser 10maintains a constant pressure in the liquid in the mould through therunners 4 and risers 20. When the mould is fully inverted the pressureis maintained by pressure in runners 4 and risers 20.

Hence the casting operation is faster than previous systems and as themolten metal pressure is maintained during the solidification of themetal by virtue of the pressure riser maintaining the pressure duringthe inversion of the mould, the mould can be removed from the machineand a further mould position for casting.

The total cycle time from load to unload of the mould is estimated to beapproximately three minutes. If there are multiple casting machines on aturntable arrangement there is a potential of producing a casting everythirty seconds.

It is considered therefore that the means and processes relating to themould pressure riser such as that described, herein would prove to be ofconsiderable benefit to those using counter gravity techniques to castaluminium in particular.

Although the invention has been described in some detail the inventionis not to be limited hereto but can include variations and modificationsfalling within the spirit and scope of the invention.

1. A sand mould for making a metal casting; comprising: a mould cavityhaving a shape corresponding to that of the metal casting, a laundersection for engagement with a liquid metal filling system, primaryrisers connecting the launder section to the mould cavity, sealing meansfor disengagement from the liquid metal filling system when the mouldcavity has been filled, and a pressure riser connected to the laundersection and the primary risers; wherein, when the sand mould is in afilling position, the primary risers extend upwardly to the mould cavityand the pressure riser extends upwardly outside the mould cavity and isfilled with liquid metal along with mould cavity; and wherein, when thesand mould is rotated 180° from its filling position to an invertedposition, the pressure riser maintains a constant pressure in the liquidmetal in the mould cavity through the primary risers.
 2. A sand mould asset forth in claim 1, wherein a chill section defines a part of themould cavity.
 3. A sand mould as set forth in claim 2, wherein the chillsection defines a top part of the mould cavity when the sand mould is inits filling position.
 4. A sand mould as set forth in claim 1, furthercomprising runners extending between the launder section and the primaryrisers.
 5. A sand mould as set forth in claim 1, wherein the sealingmeans comprises a sand slide that closes to disengage the liquid metalfilling system.
 6. A sand mould as set forth in claim 1, wherein thepressure riser has a height equal to the height of the mould cavity whenthe sand mould is in its filling position.
 7. In combination, the sandmould of claim 1, and a liquid metal filling system engaged with thelaunder section.
 8. The combination set forth in claim 7, wherein theliquid metal filling system comprises a holding furnance for the liquidmetal and a pump for pumping the liquid metal from the holding furnaceto the launder section of the sand mould.
 9. The combination set forthin claim 8, further comprising a PLC controlled closed loop feedback ofmetal liquid fill rate.
 10. The combination set forth in claim 9,wherein the pump is controlled by the PLC.
 11. The combination set forthin claim 10, wherein the sealing means is controlled by the PLC.
 12. Thecombination set forth in claim 9, wherein the sealing means iscontrolled by the PLC.
 13. The combination set forth in claim 7, furthercomprising a roll over fixture for rotating the sand mould through 180°from the filling position to the inverted position while the metal inthe sand mould is still liquid.
 14. The combination set forth in claim13, wherein the sand mould is connected to the roll over fixture via achill section.
 15. The combination set forth in claim 14, wherein thechill section forms a part of the mould cavity.
 16. The combination setforth in claim 15, wherein the chill section forms a top part of themould cavity when the sand mould is in its filling position.
 17. Aprocess for counter gravity sand casting with the sand mould of claim10, said process comprising the steps of: placing the sand mould in itsfilling position; engaging the launder section with the liquid metalfiling system; filling the mould cavity with liquid metal while the sandmould is in its filling position, thereby also filling the primaryrisers and the pressure riser; sealing the sand mould via the sealingmeans; and rotating the sand mould 180° to its inverted position;wherein, during the rotating step, the pressure riser maintains aconstant pressure in the liquid metal in the mould cavity through theprimary risers.
 18. A process as set forth in claim 17, wherein, thefilling, sealing, and rotating step are performed at a filling locationand wherein the sand mould is removed from this filling location to asolidification location after said rotating step.
 19. A process as setforth in claim 18, wherein the sand mould is loaded at the fillingstation and wherein the metal casting is unloaded from the sand mould atthe solidification location.
 20. A process as set forth in claim 19wherein the cycle time from load to unload is approximately threeminutes.
 21. A process as set forth in claim 19, wherein multiplefilling locations are positioned in a turntable arrangement.